1. Field of the Invention
The invention relates to a sheet feeder for separating sheets of paper, cardboard, plastic and the like from a stack and for feeding the sheets in a sheet travel direction in a sheet travel plane to a downstream sheet processing machine. The sheet feeder includes a first suction wheel as viewed in the sheet travel direction and a second suction wheel as viewed in the sheet travel direction. The first suction wheel is disposed above the stack and the second suction wheel is disposed downstream of the stack and above or below the sheet travel plane.
2. Description of the Related Art
Buckle folders are known in the art. The structure of a buckle folder including a plurality of buckle folding units is disclosed in German Patent Application DE 10 2004 041 471 A1. A buckle folding unit is formed of a buckle plate and three folding rollers disposed to form two pairs of folding rollers. Knife folders are likewise known in the art. A single-knife folding unit for folding printed and pre-folded sheets is known from German Patent Application DE 29 40 360 A1. Combination folders combine bucket folding units and knife folding units. In a first folding station, parallel folds are created in buckle folding units and in a downstream folding station, cross folds are created in knife folding units. German Patent Application DE 10 2006 055 301 A1 discloses combination folders including a plurality of buckle folding units and downstream knife folding units.
Known sheet feeders for sheet-fed printing presses and so-called stack feeders for sheet-fed folders include a suction head equipped with combined lifting/dragging suction elements for separating sheets and accelerating separated sheets. A disadvantage of such alternating systems is that the dragging suction element carries out a reciprocating movement and can only transport a sheet during the advancing movement. No sheet can be transported during the return movement, which is a resetting movement of the dragging suction element and represents dead time. If the number of cycles of the sheet feeder is to be increased, i.e. if the throughput of the sheet feeder is to be improved and more sheets are to be fed per unit of time, the speed of the advancing and return movement of the dragging suction unit needs to be increased. The speed increase of the dragging suction unit is mechanically limited due to the translatory, alternating sequence of motions.
A common way of increasing the throughput or productivity of folders is to increase the speed at which the sheets are transported through the folder. An increased speed causes deformations and damage to a respective signature, which may result in transportation problems and a significant deterioration of the quality of the final products.
German Patent Application DE 10 2008 048 287 A1, corresponding to U.S. Patent Application Publication No. 2010/0075821, discloses a sheet feeder for a folder that includes a suction element for lifting and separating sheets in the area of the rear edge of a stack of sheets and a suction wheel in the front area of the stack. An adjustment of the position of the suction wheel permits modification of the degree of overlap between the sheets that are transported over a downstream transport table to a downstream folder. More overlap results in a greater throughput without a need to increase the sheet speed, thus avoiding any negative effect on the quality of the signatures.